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When the particle is considered to be infinitely light or have an infinitesimal charge, it is said that we deal with a probe and the back-reaction is neglected. However, a real object also carries a mass and charges itself.
They modify the original environment (for example, they help to curve the space in general relativity) and this modification - the back-reaction - has to be taken into account when a more accurate calculation is performed.
In Cosmology the term Back-reaction is used for the measure of the non commutativity of the averaging procedure and the dynamical evolution of space-time. The existence of an isotropy scale is determined by the length scale at which the Back-reaction parameter vanishes. The existence of such scale still needs experimental confirmation.
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